The Gorlice–Tarnów Offensive during World War I started as a minor German offensive to relieve Russian pressure on the Austro-Hungarians to their south on the Eastern Front, but resulted in the total collapse of the Russian lines and their retreat far into Russia. The continued series of actions lasted the majority of the campaigning season for 1915, starting in early May and only ending due to bad weather in October.

When the actions petered out in late September, the vast majority of two Russian armies had been destroyed, and all Russian forces had been ejected from the Masurian Lakes area of modern north-east Poland losing almost 200,000 soldiers (killed or taken into captivity).

Things were not going so well to their south, however. Here the bulk of the Russian army faced an equally large group of Austro-Hungarian units, who started their own offensive in late August, and initially pushed the Russians back well into what is now central Poland. However, a well-executed Russian counterattack in late September pushed them back over their own borders in disarray, allowing the Russians to start the Siege of Przemyśl.

The Germans came to their aid by forming up the 9th Army and attacking during the Battle of the Vistula River. Although it was initially successful, the attack eventually petered out and the Germans returned to their starting points.

The Russians followed up by redeploying their armies for a further offensive into Silesia, placing both Austria and Germany at risk. When the Central Powers heard of this, the 9th Army was redeployed to the north, allowing them to put serious pressure on the Russian right flank in what developed as the Battle of Łódź in early November. The Germans failed to encircle the Russian units, and the battle ended inconclusively with an orderly Russian withdrawal to the east near Warsaw. Weather prevented further actions over the next months.

General Franz Conrad von Hötzendorf, the Chief of Staff of the Austro-Hungarian Army, originally proposed the idea of breaking up the front line in the area of Gorlice. At first this idea was rejected by German Chief of Staff, Erich von Falkenhayn, who believed that the fate of the war depended on the western front. Later he changed his mind and decided for a major offensive in the Gorlice-Tarnów area, south-east of Krakow, at the far southern end of the Eastern Front.

General Mackensen had been given command of both German and Austro-Hungarian forces, and on 2 May, after a heavy artillery bombardment, he launched an attack which caught the Russians by surprise. He concentrated 10 infantry and one cavalry division (126,000 men, 457 light, 159 heavy pieces of artillery and 96 mortars) on the 35 km (22 mi) of the breakthrough sector of the front line against five Russian divisions (60,000 men with 141 light and four heavy pieces of artillery).

The Central Powers shattered the Russian defenses, and the Russian lines collapsed. Radko-Dimitrejew quickly sent two divisions against the Austro-German breakthrough, but being ill-prepared they were utterly annihilated without being able to report back to their headquarters. From Russian point of view, both divisions simply disappeared from the map.

Russian prisoners of war after the battle.

The Russian III Army left in enemy hands about 140,000 prisoners, and almost ceased to exist as a fighting unit. The 3rd Caucasian Corps, for example, brought up to establishment of 40,000 men in April, found itself reduced to 8,000. It was thrown into the battle on the San against the Austrian I Army, and succeeded in taking some 6,000 prisoners and nine guns. One division was down to 900 men on 19 May.

The Russians were forced to withdraw, the Central Powers recaptured most of Galicia, and the Russian threat to Austria-Hungary was averted. Particularly gratifying was the recapture of Przemyśl on 3 June. The same day, fresh offensives were launched: the Austrian IV and VII armies on the flank of the XI Army aiming for the River Dniester.

By 17 June, the defenders had pulled back on Lwów (later Lvov),(now Lviv) the capital of Galicia, and on the 22nd Austria-Hungary's fourth largest city was recaptured. With this loss, which meant that most of Galicia had returned to Austrian hands, the lines stabilized in the south. The penetration progressed about 160 km (99 mi) at its deepest, reducing the Polish salient to perhaps ⅓ of its pre-war size.

Trying to save Russian forces from suffering heavy casualties and gain time needed for the massive buildup of war industries at home, the Russian Stavka decided to gradually evacuate Galicia and the Polish salient to straighten out the frontline. A strategic retreat was initiated, which is known as the Great Retreat of 1915.

Warsaw was evacuated and fell on 4 August to the new German 12th Army. At the end of the month Poland was entirely in Austro-German hands, and 750,000 Russian prisoners had been taken.